Once the story really splits after all of book 3, they're going to have a ton of ground to cover as everyone gets spread out. It's gonna slow the overall telling down quite a bit. The problem is gonna come after that, presuming book 6 comes out in the next two years.

Assuming book 6 would be seasons 7 and 8, that only leaves him about 5 years to complete book 7.

There is quite a lot of spread out characters already and book two got squeezed into one season. I can see a lot of Greyjoy and Dorne material being tossed and a condensing of redundant scenes at the Wall and on the riverboat.